1. Field of the Invention
The present invention relates to a charge/discharge control circuit for detecting a voltage and an abnormality of a secondary battery, and a battery device including the charge/discharge control circuit. In particular, the present invention relates to a charge/discharge control circuit that includes a built-in level shifter circuit for converting a low voltage signal to a high voltage signal for positive and negative terminals, and a battery device including the charge/discharge control circuit.
2. Description of the Related Art
FIG. 4 illustrates a circuit diagram of a related-art battery device including a charge/discharge control circuit. The related-art battery device including the charge/discharge control circuit includes a secondary battery 1, a control circuit 2 for monitoring a voltage of the secondary battery 1, PMOS transistors 401, 408, and 409, NMOS transistors 402 and 410, an inverter 407, a switch circuit 3 for controlling charge and discharge of the secondary battery 1, an external terminal 4 and an external terminal 5 to be connected to a charger 8 or a load 9, a level shifter circuit 74, a pull-down resistor 411, a VDD terminal, a VSS terminal, a charge control terminal 13, a discharge control terminal 12, and an overcurrent detection terminal 14.
The level shifter circuit 74 includes PMOS transistors 403 and 405 and NMOS transistors 404 and 406. The switch circuit 3 includes a discharge control switch 10 and a charge control switch 11.
The control circuit 2 monitors the voltage of the secondary battery 1. When the charger 8 is connected and the battery voltage of the secondary battery 1 becomes an overcharged state, the control circuit 2 outputs a signal of High so that the discharge control switch 10 may be turned on, and outputs a signal of High so that the charge control switch 11 may be turned off. In this way, a charge current from the charger 8 is interrupted to stop the charge to the secondary battery 1. When the load 9 is connected and the voltage of the secondary battery 1 becomes an overdischarged state, the control circuit 2 outputs a signal of Low so that the discharge control switch 10 may be turned off, and outputs a signal of Low so that the charge control switch 11 may be turned on. In this way, a discharge current to the load 9 is interrupted to stop the discharge of the secondary battery 1.
As described above, when the secondary battery 1 is overcharged or overdischarged, the discharge control switch 10 or the charge control switch 11 is controlled, thereby being capable of stopping the charge current or the discharge current (see, for example, FIG. 1 of Japanese Patent Application Laid-open No. Hei 10-225007).
In the related art, however, there is a problem in that, when a transistor having a low gate breakdown voltage is used for the level shifter circuit 74, the transistor may be broken when the charger is connected. Further, there is another problem in that, when a MOS transistor having a high breakdown voltage structure is used in order to prevent the breakdown at the time of connection of the charger, the area of the MOS transistor may increase to increase a layout area.